All animals possess a “nose,” an olfactory sense organ that allows for the recognition and discrimination of chemosensory information in the environment. Humans, for example, are thought to recognize over 10,000 discrete odors with exquisite discriminatory power such that subtle differences in chemical structure can often lead to profound differences in perceived odor quality. What mechanisms have evolved to allow the recognition and discrimination of complex olfactory information and how is olfactory perception ultimately translated into appropriate behavioral responses? The recognition of odors is accomplished by odorant receptors that reside on olfactory cilia, a specialization of the dendrite of the olfactory sensory neuron. The odorant receptor genes encode novel serpentine receptors that traverse the membrane seven times. In several vertebrate species, and in the invertebrate Caenorhabditis elegans, as many as 1000 genes encode odorant receptors, suggesting that 1–5% of the coding potential of the genome in these organisms is devoted to the recognition of olfactory sensory stimuli (Buck and Axel, 1991; Levy et al., 1991; Parmentier et al., 1992; Ben-Arie et al., 1994; Troemel et al., 1995; Sengupta et al., 1996; Robertson, 1998). Thus, unlike color vision in which three photoreceptors can absorb light across the entire visible spectrum, these data suggest that a small number of odorant receptors are insufficient to recognize the full spectrum of distinct molecular structures perceived by the olfactory system. Rather, the olfactory sensory system employs an extremely large number of receptors, each capable of recognizing a small number of odorous ligands.
The discrimination of olfactory information requires that the brain discern which of the numerous receptors have been activated by an odorant. In mammals, individual olfactory sensory neurons express only one of a thousand receptor genes such that the neurons are functionally distinct (Ngai et al., 1993; Ressler et al., 1993; Vassar et al., 1993; Chess et al., 1994). The axons from olfactory neurons expressing a specific receptor converge upon two spatially invariant glomeruli among the 1800 glomeruli within the olfactory bulb (Ressler et al., 1994; Vassar et al., 1994; Mombaerts et al., 1996; Wang et al., 1998). The bulb therefore provides a spatial map that identifies which of the numerous receptors has been activated within the sensory epithelium. The quality of an olfactory stimulus would therefore be encoded by specific combinations of glomeruli activated by a given odorant.
The logic of olfactory discrimination is quite different in the nematode, C. elegans. Despite the large size of the odorant receptor gene family, volatile odorants are recognized by only three pairs of chemosensory cells each likely to express a large number of receptor genes (Bargmann and Horvitz, 1991; Colbert and Bargmann, 1995; Troemel et al., 1995). Activation of any one of the multiple receptors in one cell will lead to chemoattraction, whereas activation of receptors in a second cell will result in chemorepulsion (Troemel et al., 1997). The specific neural circuit activated by a given sensory neuron is therefore the determinant of the behavioral response. Thus, this invertebrate olfactory sensory system retains the ability to recognize a vast array of odorants but has only limited discriminatory power.
Vertebrates create an internal representation of the external olfactory world that must translate stimulus features into neural information. Despite the elucidation of a precise spatial map, it has been difficult in vertebrates to discern how this information is decoded to relate the recognition of odors to specific behavioral responses. Genetic analysis of olfactory-driven behavior in invertebrates may ultimately afford a system to understand the mechanistic link between odor recognition and behavior. Insects provide an attractive model system for studying the peripheral and central events in olfaction because they exhibit sophisticated olfactory-driven behaviors under control of an olfactory sensory system that is significantly simpler anatomically than that of vertebrates (Siddiqi, 1987; Carlson, 1996). Olfactory-based associative learning, for example, is robust in insects and results in discernible modifications in the neural representation of odors in the brain (Faber et al., 1998). It may therefore be possible to associate modifications in defined olfactory connections with in vivo paradigms for learning and memory.
Olfactory recognition in the fruit fly Drosophila is accomplished by sensory hairs distributed over the surface of the third antennal segment and the maxillary palp. Olfactory neurons within sensory hairs send projections to one of 43 glomeruli within the antennal lobe of the brain (Stocker, 1994; Laissue et al., 1999). The glomeruli are innervated by dendrites of the projection neurons, the insect equivalent of the mitral cells in the vertebrate olfactory bulb, whose cell bodies surround the glomeruli. These antennal lobe neurons in turn project to the mushroom body and lateral horn of the protocerebrum (reviewed in Stocker, 1994). 2-deoxyglucose mapping in the fruit fly (Rodrigues, 1988) and calcium imaging in the honeybee (Joerges et al., 1997; Faber et al., 1998) demonstrate that different odorants elicit defined patterns of glomerular activity, suggesting that in insects as in vertebrates, a topographic map of odor quality is represented in the antennal lobe. However, in the absence of the genes encoding the receptor molecules, it has not been possible to define a physical basis for this spatial map.
The present application discloses a large family of genes that are likely to encode the odorant receptors of Drosophila melanogaster. Difference cloning, along with analysis of Drosophila genomic sequences, has led to the identification of a novel family of putative seven transmembrane domain receptors likely to be encoded by 100 to 200 genes within the Drosophila genome. Each receptor is expressed in a small subset of sensory cells (0.5–1.5%) that is spatially defined within the antenna and maxillary palp. Moreover, different neurons express distinct complements of receptor genes such that individual neurons are functionally distinct. Identification of a large family of putative odorant receptors in insects indicates that, as in other species, the diversity and specificity of odor recognition is accommodated by a large family of receptor genes. The identification of the family of putative odorant receptor genes may afford insight into the logic of olfactory perception in Drosophila. 
Insects provide an attractive system for the study of olfactory sensory perception. The present application identifies a novel family of seven transmembrane domain proteins, encoded by 100 to 200 genes, that is likely to represent the family of Drosophila odorant receptors. Members of this gene family are expressed in topographically defined subpopulations of olfactory sensory neurons in either the antenna or the maxillary palp. Sensory neurons express different complements of receptor genes, such that individual neurons are functionally distinct. The isolation of candidate odorant receptor genes along with a genetic analysis of olfactory-driven behavior in insects may ultimately afford a system to understand the mechanistic link between odor recognition and behavior.